1. Field of the Invention
This invention relates to the design of operational amplifiers and, in particular, to the design of the output stage of operational amplifiers intended to provide an output voltage which switches (or alternates) between a first output reference voltage and a second output signal voltage level. The signal voltage may be time varying.
2. Description of the Prior Art
The use of operational amplifiers to switch between output voltage levels is known. Such operational amplifiers require a specific amount of time to make the transition from one output voltage level to another output voltage level. The required time interval is commonly divided into two sub-intervals, the slewing time interval and the settling time interval.
The rate at which the output voltage of an operational amplifier changes from a first level to a second level during the slewing time interval is called the slew rate dV/dt. The slew rate is limited to a maximum value determined by the input stage bias current I.sub.1 and the size of the internal stabilization capacitor (Cs). The maximum value of the slew rate is known as the first-stage slew rate limit and may be expressed as dV/dt=I.sub.1 /Cs. In presently known operational amplifiers which are required to switch from a first voltage level to a second voltage level, the first voltage level may be a reference level such as ground or the offset voltage of the differential input stage of the operational amplifier. The second voltage level may be a specific time varying voltage level which the operational amplifier is required to provide at its output. Thus, as the value of the time varying voltage increases, it takes the operational amplifier longer to slew from the ground or reference voltage level to the level of the second or time varying voltage. The longer the time required to slew to the desired voltage level, the less time is available (during a given clock period) for the settling time interval, during which the amplifier output voltage may settle to a value approaching that of the desired output voltage.